This invention relates to improvements protective relays to be used for protecting electric power transmission lines and the like, and more particularly to improvement in a digital distance relay wherein the sensitivity for detecting faults is substantially improved.
The capability of detecting faults with high sensitivity in an electric system to be protected is essential to a protective relay. More specifically, when a long-distance electric power transmission line is protected by a distance relay, it is essential to accurately determine the distance between the position of the relay and a faulty point even when the fault current is small. The capability of determining the distance is hereinafter termed "distance detecting property". The sensitivity and the distance detecting property of ordinary protective relays will now be described with regard to a mho relay, for instance.
The mho characteristics shown in FIG. 1 can be realized in various ways utilizing, for instance, a protective relay of an electromagnetic induction type, the basic operation of which can be defined by the following well-known relation. EQU K.sub.Z V.multidot.I cos (.theta.-.phi.)-V.sup.2 -K.sub.o &gt;0 (1)
wherein: K.sub.Z is a predetermined constant, .theta. is a phase angle between an input voltage V and an input current I both detected by the relay, K.sub.o is a constant as described hereinafter, and .phi. is an electric angle at which a maximum sensitivity of the relay can be obtained.
Depending on the constant K.sub.o, the sensitivity of the protective relay can be adjusted suitably. However, the effect of the constant K.sub.o for adjusting the sensitivity is substantially reduced when the input voltage V or the input current I is comparatively high.
The distance detecting property of the protective relay, while operating at a maximum-sensitivity angle .theta. (that is .theta.=.phi. and cos (.theta.-.phi.)=1), can be expressed as follows in terms of the input current I of the expression (1). EQU I.sup.2 &gt;K.sub.o /Z(K.sub.Z -Z) (2)
where Z=V/I.
FIG. 2 represents this relation (2). In FIG. 2, a reference character A designates a point at which the distance detecting property of the relay is 90% of the maximum protectable range of the mho relay, and a reference character B designates a point defining a minimum operable input current. It is apparent from the relation (2) that the currents I.sub.A and I.sub.B corresponding respectively to those at points A and B in FIG. 2 are increased in accordance with the constant K.sub.o, and that the sensitivity of the protective relay is thereby reduced. In other words, it is apparent that a smaller value of the constant K.sub.o is preferably because the sensitivity and the distance detecting property of the protective relay are thereby improved.
However, a simple reduction of the constant K.sub.o causes disadvantages in the operation of the protective relay in which the relay tends to pick-up noise contained in the input AC voltage and current, particularly when the voltage and current are in their reduced ranges, or in which the relay is disturbed by offset and drift in the input or by a quantizing error when the relay is of a digital type.
In order to eliminate these disadvantages, a comparatively large value must be selected for the constant K.sub.o, which inevitably increases the current value corresponding to the point A, and lowers the distance detecting property of the protective relay in a lower current range.